added --customIndexTemplate option

[MananJethwani]

@kelson42 this PR will add --customIndexTemplate argument.

This depends on kiwix/libkiwix#607

[kelson42]

-c probably does not work because https://github.com/kiwix/kiwix-tools/pull/477/files#diff-29eedb326cc54203c610d3910176face715ff30664857e7d8ced287ff455551aL122 has not been modified.

[MananJethwani]

@kelson42 sorry, it wasn't working when I used -c it was working with --c, my bad.

[kelson42]

The usage is not aligned properly. I would probably call the option only --customIndex.

If I give an invalid template file, then it should not start and print an appropriate error. For the moment, $ kiwix-serve --port=8080 --customIndexTemplate=phzh_core-greek-one_el_2020-12.zim phzh_core-greek-one_el_2020-12.zim starts and gives a blanck index page.

Same remark for a non-existing path.

Same remark for non-readable file.

[MananJethwani]

@kelson42 the error for the non-readable file should be propagated by kiwix::getFileContent() in libkiwix/tools.h, so I couldn't do much but I am showing a warning if unreadable or empty template file is passed.
I have added errors for all other cases and exiting the code.

[kelson42]

@MananJethwani I don't know the code detail, but it should stop, not display a warning. Probably kiwix::getFileContent() should trigger an exception and you should catch it.

[mgautierfr]

Agree with @kelson42 if the user pass a option and we cannot respect it, it should be a error.

[mgautierfr]

Two really easy fix to do. But we are good.

This PR commits could be squash together in only one.

Need to kiwix/libkiwix#607 to be merged first.

[mgautierfr]

We are good.
To merge once kiwix/libkiwix#607 is merged.